Real-time sensing of a brain lesion under operation on the brain is investigated. We selected a combination of the functional magnetic particles and small and high sensitive magnetic sensor, the magneto-impedance (MI) sensor, for the sensing method. At first, we detected a magnetic field of an agarose gel containing magnetite particles using the MI sensor. It was found that a magnetization of the magnetite by a static magnetic field is necessary for the detection. We also tried to detect the magnetite particles in a rat tumor which was transplanted under the skin of the rat leg. The magnetite cationic liposomes, at which the magnetite particles were coated with a cationic lipid and phospholipids membrane, were directly injected in the tumor and magnetized by the static magnetic field generator at 1.8 T. By using the MI sensor, the magnetic field distribution of the MCL was detected, and its distribution indicated that the MCL is localized in the tumor.